Re: [RFC] Restrict size of page_cgroup->flags

[Daisuke Nishimura]

On Wed, 6 Oct 2010 19:53:14 +0530
Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:

> I propose restricting page_cgroup.flags to 16 bits. The patch for the
> same is below. Comments?
> 
> 
> Restrict the bits usage in page_cgroup.flags
> 
> From: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> 
> Restricting the flags helps control growth of the flags unbound.
> Restriciting it to 16 bits gives us the possibility of merging
> cgroup id with flags (atomicity permitting) and saving a whole
> long word in page_cgroup
> 
I agree that reducing the size of page_cgroup would be good and important.
But, wouldn't it be better to remove ->page, if possible ?

Thanks,
Daisuke Nishimura.

> Signed-off-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> ---
> 
>  include/linux/page_cgroup.h |    3 +++
>  mm/page_cgroup.c            |    1 +
>  2 files changed, 4 insertions(+), 0 deletions(-)
> 
> 
> diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
> index 872f6b1..10c37b4 100644
> --- a/include/linux/page_cgroup.h
> +++ b/include/linux/page_cgroup.h
> @@ -44,8 +44,11 @@ enum {
>  	PCG_FILE_WRITEBACK, /* page is under writeback */
>  	PCG_FILE_UNSTABLE_NFS, /* page is NFS unstable */
>  	PCG_MIGRATION, /* under page migration */
> +	PCG_MAX_NR,
>  };
>  
> +#define PCG_MAX_BIT_SIZE	16
> +
>  #define TESTPCGFLAG(uname, lname)			\
>  static inline int PageCgroup##uname(struct page_cgroup *pc)	\
>  	{ return test_bit(PCG_##lname, &pc->flags); }
> diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
> index 5bffada..e16ad2e 100644
> --- a/mm/page_cgroup.c
> +++ b/mm/page_cgroup.c
> @@ -258,6 +258,7 @@ void __init page_cgroup_init(void)
>  	unsigned long pfn;
>  	int fail = 0;
>  
> +	BUILD_BUG_ON(PCG_MAX_NR >= PCG_MAX_BIT_SIZE);
>  	if (mem_cgroup_disabled())
>  		return;
>  
> 
> -- 
> 	Three Cheers,
> 	Balbir
> 
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